The United States has new evidence showing that the government of Syria used the nerve gas sarin against its own people, Secretary of State John Kerry said Sunday.

Kerry said on news programs that the U.S. has received hair and blood samples that have "tested positive for signatures of sarin," a deadly nerve agent first developed in Germany in the 1930s.

"In the last 24 hours, we have learned through samples that were provided to the United States and that have now been tested from first responders in East Damascus, and hair samples and blood samples have tested positive for signatures of sarin," Kerry said on NBC's Meet the Press.

On CNN's State of the Union, Kerry laid out the same case, and noted that the new evidence came not from United Nations inspectors but from independent channels. Kerry said the new evidence boosts President Obama's call for Congress to approve military action against Syria. "I believe the case for action will get stronger by the day."

The administration continues to avoid saying that there is 100% proof of its case against Syrian President Bashar Assad. "The word 'slam-dunk' should be retired from American national security issues," Kerry told NBC host David Gregory.

"We are saying that the high confidence that the intelligence community has expressed and the case that I laid out the other day is growing stronger by the day," Kerry said on NBC. "We know where this attack came from. We know exactly where it went. We know what happened exactly afterwards."

The U.S. Centers for Disease Control and Prevention describes sarin as "a human-made chemical warfare agent classified as a nerve agent. Nerve agents are the most toxic and rapidly acting of the known chemical warfare agents." Sarin was developed as a pesticide, and it works by blocking an enzyme that serves as the "off-switch" for the body's glands and muscles, sending normal bodily systems into constant activity. Eventually victims can no longer breathe.

On Saturday, President Obama said he had decided the United States should launch a military attack against Syria in retaliation for an Aug. 21 chemical weapons attack in the Damascus suburbs. But he said he would first ask Congress to approve the use of military force.